Optical exposure unit for electrophotographic printing device

ABSTRACT

An electrophotographic printing device is provided with an exposure unit comprising light-emitting diodes which are light sources and means for transmitting the light beams emitted from the light-emitting diodes and converging or focusing them like converging lenses, whereby electrostatic latent images of characters and the like are formed on the surface of a photosensitive member.

BACKGROUND OF THE INVENTION

The present invention relates to an electrophotographic printing deviceof the type having an exposure unit comprising the combination of aplurality of light-emitting diodes which are light sources and aplurality of means for transmitting the light beams emitted from thelight-emitting diodes and converging or focusing them like converginglenses (to be referred to as "the light-beamtransmission-and-convergence means" in this specification). The lastmentioned optical means may be, for instance, the products of NipponSheet Glass Co., Ltd. sold under the trademark of "SELFOC". This opticalmeans is described in "Optical Characteristics of a Light-Focusing FiberGuide and Its Applications", Uchida, et al, IEEE Journal of QuantumElectronics, Volume QE-6, No. 10, Page 606, Oct. 1970.

The prior art electrophotographic printing devices of the type having anexposure unit comprising a cathode-ray tube and an optical fiber plate(a plurality of optical fibers are arranged in a plate form) have beenwell known. The electron beam emitted from the cathode-ray tube istransmitted through the optical fiber plate so as to be focused on thesurface of a photosensitive member. However, the optical fiber plateonly has a function of transmitting the light beam therethrough and oneend of the optical fiber plate is desirable to be made into contact withthe surface of the photosensitive member. Furthermore, the optical fiberplate must be moved relative to the surface of the photosensitivemember, so that the optical fiber plate must be spaced apart from thesurface of the photosensitive member in order to avoid damage to thesurface of the photosensitive member by friction. Thus, the prior artexposure unit must satisfy the above two requirements which arecontradictory to each other. As a result, with the prior art exposureunit, a distance of from 50 to 100 μm must be maintained with a highdegree of accuracy between the optical fiber plate and the surface ofthe photosensitive member. Therefore, the prior art exposure unit needsa mechanism for supporting the optical fiber plate which is complex inconstruction and which must support the optical fiber plate with a highdegree of accuracy. As a result, the prior art exposure unit is veryexpensive. In addition, toner tends to adhere to the end surface of theoptical fiber plate, so that exposures are adversely affected andsubsequently the electrostatic latent images are degraded or deformed.Moreover, the use of a cathode-ray tube results in an increase in sizeof the exposure unit.

In order to make the exposure units compact in size, there has beenproposed the use of light-emitting diodes. With the light-emittingdiodes, the exposure units can be made 1/10 in size as compared with theexposure units using a cathode-ray tube. However, as with thecathode-ray tube exposure units, the distance between the end of theoptical fiber plate and the surface of a photosensitive member must bestrictly maintained. Thus, the light-emitting diode type exposure unitsstill need a complex and expensive mechanism for supporting the opticalfiber plate.

SUMMARY OF THE INVENTION

In view of the above, one of the objects of the present invention is toprovide an electrophotographic printing device in which an exposure unitcomprises a plurality of light-emitting diodes and a plurality oflight-beam transmission-and-convergence means so that the focusing orconverging power of the latter is effectively utilized so as to permitthe inner end of the light-beam transmission-and-convergence means to bespaced apart from the surface of a photosensitive member by a relativelong distance as compared with the prior art exposure units, whereby theprior art complex and precision optical fiber plate supportingmechanisms can be eliminated and consequently the exposure unit can bemade compact in size and light in weight and inexpensive to fabricate.

According to a second embodiment of the present invention, a pluralityof light-beam transmission-and-convergence means and a plurality oflight-emitting diodes are disposed in a zig-zag form, respectively, inthe direction perpendicular to the direction of the movement of thephotosensitive member, whereby uniform exposures can be ensured andconsequently high quality images can be formed.

According to a third embodiment of the present invention, a plurality ofarrays each comprising a predetermined number of light-emitting diodesare disposed at an angle to the direction perpendicular to the directionof the movement of the photosensitive member, whereby the pictureelement density can be increased and consequently the resolution can beimproved.

According to a fourth embodiment of the present invention, the movingspeed of the photosensitive member is taken into consideration.Therefore, the light emitting aperture of each light-emitting diode ismade into the form of an ellipse whose major axis is extended in thedirection perpendicular to the movement of the photosensitive member,whereby an electrostatic latent image focused on the photosensitivemember consists of picture elements which are almost circular andconsequently the image quality can be remarkably improved.

According to a fifth embodiment, either or both of an array oflight-emitting diodes and an array of light-beamtransmission-and-convergence means are moved toward or away from thesurface of the photosensitive member by an extremely small distance,whereby the diameter of each picture element can be arbitrarilyselected.

The above and other objects, effects and features of the presentinvention will become more apparent from the following detaileddescription of preferred embodiments thereof taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first embodiment of an exposure unit in accordance withthe present invention;

FIG. 2 is a view used for the explanation of the printing or recordingprocess by an electrophotographic printing device incorporating theexposure unit shown in FIG. 1;

FIG. 3 is a view used for the explanation of the relationship betweenthe incidence angle to a light-beam transmission-and-convergence meansof the light beam emitted from a light-emitting diode and theillumination obtained on the surface of a photosensitive member by thelight-beam transmitted through and focused by the light-beamtransmission-and-convergence means;

FIG. 4 is a schematic perspective view of a second embodiment of thepresent invention;

FIG. 5 shows the position relationships between the light-emittingdiodes and the light-beam transmission-and-convergence means in thesecond embodiment;

FIG. 6 is a diagram of a timing circuit used in the second embodiment;

FIG. 7 shows an arrangement of light-emitting diode chips;

FIG. 8 is a schematic perspective view of a third embodiment of thepresent invention;

FIG. 9A is a view used for the explanation of the relationship between astraight array of light-emitting diodes and a resulting array of spotsof light or picture elements;

FIG. 9B is a view used for the explanation of the relationship betweeninclined arrays of light-emitting diodes and a resulting array of spotsof light or picture elements;

FIG. 10 is a diagram of a timing circuit used in the third embodiment;

FIG. 11 is a diagram of a driver circuit used in the third embodiment;

FIG. 12 is a schematic perspective view of a fourth embodiment of thepresent invention;

FIG. 13A shows a tone image obtained when each of the light-emittingdiodes has a circular light-emitting aperture;

FIG. 13B shows a tone image obtained when each of the light-emittingdiodes has an elliptical light-emitting aperture in accordance with thepresent invention;

FIG. 14 shows an image (c) obtained by the fifth embodiment incomparison with the images (A) and (B) obtained by theelectrophotographic printing device in which the diameter of pictureelements is not variable;

FIG. 15A is a schematic front view of a fifth embodiment of the presentinvention;

FIG. 15B is a side view thereof;

FIG. 16 shows a modification of the fifth embodiment; and

FIG. 17 shows another modification of the fifth embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 is shown a first embodiment of the present invention.Assembled into a unitary construction are a subassembly of a pluralityof light-emitting diodes 1 disposed at right angle to the plane of FIG.1, a subassembly of a plurality of light-beamtransmission-and-convergence means 2 disposed at a right angle to theplane of FIG. 1 and a spacer 3 interposed between the light-emittingdiode subassembly and the light-beam transmission-and-convergence meanssubassembly. The assembly is then encapsulated with a resin or the liketo provide an exposure unit 4. The light-beamtransmission-and-convergence means 2 serve not only for transmitting thelight beams emitted from the light-emitting diodes 1 but also forconverging them like converging lenses. Therefore the light beamsemitted from the light-emitting diodes 1 are transmitted through thespacer 3 into the light beam transmission-and-convergence means 2 so asto be focused upon a photosensitive medium 5. The distance between thedownstream end (that is, the end facing the photosensitive medium 5) andthe surface of the photosensitive medium 5 can be increased with anincrease in focal length of the light beam transmission-and-convergencemeans 2.

In FIG. 2 is shown an electrophotographic printing device utilizing theexposure unit 4 of the type described above. The printing or recordingprocess is as follows. Initially, the photosensitive medium 5 isuniformly imparted with the charge by a charge electrode 6 and thenexposed to the light beams emitted from the light-emitting diodes in theexposure unit 4 which are turned on depending upon a pattern to becopied, whereby an electrostatic latent image is formed over thephotosensitive medium 5. Next, toner 8 is applied by a magnet roll 7 soas to develop the latent image into a visible toner image. The tonerimage is transferred by a transfer electrode 9 over a recording sheet10. Thereafter, the remaining charge is dissipated by a dischargingelectrode 11 and the toner which still remains on the photosensitivemedium 5 is removed by a fur brush 12 and the magnet roll 7. The tonerimage transferred over the recording sheet 10 is fixed by means of afixing device (not shown).

According to the first embodiment, the exposure unit 4 can be madecompact in size and can be spaced apart from the photosensitive medium 5by a relatively great distance, so that the supporting mechanisms of theprior art electrophotographic printing devices which are complex inconstruction and expensive to fabricate can be eliminated. In addition,the distance between the exposure unit 4 and the photosensitive medium 5can be easily adjusted. Furthermore, the fabrication costs can beconsiderably reduced. Moreover, the toner can be prevented from adheringto the inner end of the exposure unit 4.

FIG. 3 shows the relationship between the angle of incidence of thelight ray incident on one end (facing the light source 13) of thelight-beam transmission-and-convergence means 2 and the illuminance ofthe image formed by the means 2. If the light source 13 has a uniformbrightness over its surface, the illuminance distribution of a spot oflight 13a formed through the means 2 on the X-Y plane may beapproximated by the spheroid 14 obtained by revolving an ellipse aboutthe Z-axis. More specifically, the image of the diameter (indicated bythe imaginary line B) of the light source 13 which is in parallel withthe axis X has the illuminance distribution as indicated by the hatchedarea 14b. In like manner, the image of the chord (indicated by theimaginary line A) which is in parallel with the imaginary line B has theilluminance distribution as indicated by the hatched area 14a.

When instead of the circular light source 13, a straight array oflight-emitting diodes 1 is placed at the same position, a straight arrayof images 15 of light-emitting diodes 1 is formed on the X-Y plane. Theilluminance of the image 15b of the center light-diode 1b (which lies onthe optical axis of the means 2) is indicated by the length of the linesegment 15b' while the illuminance of the image 15a of thelight-emitting diode 1a spaced apart radially outwardly from the centerlight-emitting diode 1b is indicated by the length of the line segment15a'. It follows therefore that the farther a light-emitting diode 1 isspaced apart from the optical axis of the light-beamtransmission-and-convergence means, the lower the illuminance of theimage thereof becomes. When such variations in illuminance of imagesfocused on the photosensitive medium occur, the picture elements arevaried in toner density, and subsequently the toner image quality isdegraded. This problem, however, can be solved according to the presentinvention as will be described below.

FIG. 4 shows a second embodiment of the present invention. One ends of aplurality of light-beam transmission-and-convergence means 2 aredisposed in opposed relationship with the surface of the photosensitivemedium 5 which rotates in one direction. Furthermore, they are arrayedin zig-zag in the direction perpendicular to the direction of movementof the photosensitive medium 5; that is, the direction in parallel withthe axis of a photosensitive drum. (In this embodiment, it suffices todescribe "in parallel with the axial direction of the photosensitivedrum", but there does exist, for instance, a sheet type photosensitivemedium which moves in one direction. Therefore, the expression of "thedirection perpendicular to the direction of movement" is used.) Theselight-beam transmission-and-convergence means 2 are gathered together toconstruct a light-beam transmission-and-convergence unit 16.

A plurality of light-emitting diodes 1 are arrayed in the horizontaldirection in opposed relationship with each end of the light-beamtransmission-and-convergence means 2. They are gathered together toconstruct a light-emitting diode unit 17. When a light-emitting diode 1is lighted, an electrostatic latent image 18 formed on the surface ofthe photosensitive medium 5.

FIG. 5 shows the detailed positional relationship between the light-beamtransmission-and-convergence means 2 and the light-emitting diodes 1. Inthis example, five light-emitting diodes 1 are disposed in one row inopposed relationship with the center portion of each of the end face ofeach light-beam transmission-and-convergence means 2. For instance, thelight-emitting diode array in the upper row is so disposed that it maybe located between the adjacent light-emitting diode arrays in the lowerrow.

In this embodiment in which the light-beam transmission-and-convergencemeans 2 and the light-emitting diodes 1 are disposed in the mannerdescribed above, even when any of the light-emitting diodes in one arrayis turned on, the illuminance of the image focused on the surface of thephotosensitive medium becomes substantially equal (because all thelight-emitting diodes are located in the vicinity of the center portionof the end face of the light-beam transmission-and-convergence means),and consequently unevenness in density of the character images or thelike can be eliminated.

However, when the light-diode arrays are disposed in two rows as shownin FIG. 4, there must be a time lag between the timing at which thelight-emitting diodes in the upper row are turned on and the timing atwhich the light-emitting diodes in the lower row are turned on so thatthe straight horizontal electrostatic latent image as shown at 18 inFIG. 4 may be formed over the cylindrical surface of the photosensitivemedium 5 which rotates in the direction indicated by an arrow.

To this end, as shown in FIG. 6 the present invention provides a circuitfor controlling the timing for lighting light-emitting diodes (to bereferred to as "the timing circuit" for brevity in this specification).The timing circuit includes a buffer memory 19 capable of storingcharacter information and a (10×n ) bit one-line buffer memory 20 whichreceives the picture element information in one line from the buffermemory 19. The number 10 represents the sum of the five light-emittingdiodes a, b, c, d and e in one array in the upper row and the fivelight-emitting diodes f, g, h, i and j in one array in the lower row(See FIG. 5) and n represents the number of light-emitting diode arraypairs in the upper and lower rows.

The timing circuit further includes an oscillator 21, a timing pulsegenerator 22, 2-1 selectors 23 for selecting the upper and lower rows,driver circuits 24, inverters 25a and 25b and transistors 26a and 26b.

In the step of forming an electrostatic latent straight horizontal lineimage as shown at 18 in FIG. 4, it is assumed that the distance betweenthe upper and lower light-emitting diode rows be l_(o) mm and thephotosensitive medium 5 rotate at a speed of l₁ mm/sec. Then thelight-emitting diodes in the lower row must be turned on l_(o) /l₁ secafter those in the upper row are turned on. Therefore, in response to asignal T₁ from the timing pulse generator 22, the 2-1 selectors 23select the picture element information in the upper row and the drivercircuits 24 cause the light-emitting diodes a through e in the upper rowto be turned on. In response to a signal T₂ after l_(o) /l₁ sec, theselectors 23 select the picture element information in the lower row andthe driver circuits 24 cause the light-emitting diodes f through j inthe lower row to be turned on. The 2-1 selectors 23 and the drivercircuits 24 must be equal in number n to the light-emitting diode arraypairs in the upper and lower rows.

In summary, according to the second embodiment, the light-emitting diodearrays are disposed in the proximity of the extension of the axis ofeach light-beam transmission-and-convergence means 2 so that a lightimage with a uniform intensity can be focused on the cylindrical surfaceof the photosensitive medium 5 and consequently the toner image with auniform density can be obtained. Thus, the image quality can be improvedconsiderably. In order to form an image of a straight horizontal lineacross the surface of the photosensitive medium, the upper and lowerarrays of light-emitting diodes are turned on and off independently ofeach other so that the loads on the power supply may be reduced.

In addition, the light-emitting diode arrays can be fabricated in theform of a tip so that the assembly is much facilitated.

In order to increase the picture element density, the light-emittingdiodes 1 may be arrayed in a zig-zag form in each chip 27 as shown at(A) in FIG. 7. When the light-emitting diode chips 27 are disposed in azig-zag form in the direction perpendicular to the direction of rotationof the photosensitive medium 5, the pitch distance p' between the lastlight-emitting diode in one chip 27 and the first light-emitting diodein the adjacent tip 27 must be equal to the pitch distance p between theadjacent light-emitting diodes in each chip 27. Then, the spots of lightand hence the toner dots or picture elements are made into intimatecontact with the adjacent ones in a straight line as shown at (B) inFIG. 7, whereby the image quality can be further improved.

In FIG. 8 is shown a third embodiment of the present invention which issubstantially similar in construction to the second embodiment shown inFIG. 4 except that the light-emitting diode arrays are inclined at anangle to the direction perpendicular to the direction of the rotation ofthe photosensitive medium 5 so that the picture element density isincreased.

When, as shown in FIG. 9A, ten light-emitting diodes are arrayed in onerow at a pitch distance of 0.111 mm over the length of 1 mm, the spotsof light (or the picture elements) 29 focused on the photosensitivemedium 5 also have the same pitch distance of 0.111 mm. The resolutionis therefore 10 dots per mm. When the ratio of the diameter of the lightemerging or emitting aperture of the light-emitting diode 1 to thediameter of the spot of light 29 focused on the photosensitive medium 5is 0.6:1, then with the light-emitting diode with the light emittingaperture diameter of 0.09 mm, the spot of light with the diameter of0.15 mm is focused. It follows therefore that in order to attain a highpicture element density, the pitch distance between the light-emittingdiodes must be reduced. However, from the technical viewpoint, it isextremely difficult to reduce the pitch distance. In addition, thereduction in pitch distance will inevitably result in increase infabrication cost. Furthermore, when it is desired to write a straighthorizontal line image on the surface of the photosensitive medium, allthe light-emitting diodes must be simultaneously turned on, so that ahigh instantaneous current flows and there arises a drawback that theload on a power supply is increased.

When, as shown in FIG. 9B, the light-emitting diodes 1 are arranged at apitch distance of 0.111 mm in an array inclined at 45° relative to theline L which indicates the direction perpendicular to the direction ofthe movement of the photosensitive medium 5, the pitch distance betweenthe adjacent picture elements 29 becomes 0.078, so that 14 pictureelements 29 are arrayed over the distance of 1 mm. The resolution is,therefore, increased to 14 dots per mm.

In order to form the picture elements 29 in a straight line as shown inFIG. 9B with such inclined light-emitting diode arrays, thelight-emitting diodes 1-1, 1-2, 1-3 and so on in each array must besequentially turned on in synchronism with the moving or rotating speedof the photosensitive medium 5.

To this end, the present invention provides a timing circuit as shown inFIG. 10 and driver circuits as shown in FIG. 11. The timing circuitincludes a buffer memory 30 for storing character information and a(14×n ) bit one-line buffer memory 31 which receives the picture elementinformation in one line from the buffer memory 30. The number 14represents the number of light-emitting diodes in one array and nrepresents the number of light-emitting diode arrays.

The timing circuit further includes an oscillator 32, a sync-and-controlcircuit 33, parallel-to-serial conversion circuits 34 each of whichreceives the picture element information in one array in parallel fromthe buffer memory 31 and delivers it in serial and a shift register 35which receives a clock signal CLOCK and a start signal START from thesync-and-control circuit 33 and delivers not only the shift signal Swhich controls the parallel-to-serial conversion circuits 34 but alsosignals T₁ through T₁₄ serially.

The driver circuit shown in FIG. 11 receives the picture elementinformation DATA 1 from the parallel-to-serial conversion circuit 34 andthe timing signals T₁ through T₁₄ so as to turn on the diodes 1-1through 1-14 sequentially. The driver circuit includes 14 AND gates 36the output of which in turn are connected to respective light-emittingdiodes 1-1 through 1-14. Therefore, when one of the AND gates 36simultaneously receives the DATA 1 signal and the timing signal T, thelight-emitting diode 1 connected to the output of this AND gate isturned on. Therefore, when all AND gates 36 receive DATA 1 and thetiming signal T, the light-emitting diodes 1-1 through 1-14 aresequentially turned on in synchronism with the moving speed of thephotosensitive medium 5, the picture elements 18 are formed in astraight horizontal array as shown in FIG. 8. The driver circuit asshown in FIG. 11 must be equal in number n to the light-emitting diodearrays.

In summary, according to the third embodiment of the present invention,the pitch distance between the picture elements can be reduced withoutreducing the pitch distance between the light-emitting diodes, so thatthe resolution can be increased and subsequently the image quality canbe improved. When a straight line is printed, the light-emitting diodesare sequentially turned on as described previously, so that it is notneeded to flow a high current into the driver circuits and consequentlya low current supply can be used.

In FIG. 12 is shown a fourth embodiment of the present invention. Thelight emitting aperture 37 of each light-emitting diode is in the formof an ellipse whose major axis is perpendicular to the direction of themovement of the photosensitive medium 5, so that an almost circularpicture element 38 is formed on the medium 5. In order to clearly showthe light emitting apertures 37 and the picture elements 38, they areexaggerated in size in FIG. 12.

When the light emitting diode has a circular light emitting aperture, acircular spot of light is focused on the photosensitive medium 5, butsince the medium 5 is rotating, the picture element becomes an ellipsewhose major axis is in the direction of the movement of the medium 5. Itis assumed that the ratio in diameter between tne light emittingaperture and the spot of light be 0.6:1 and the light emitting aperturebe 0.6 d in diameter. Then, the spot of light becomes d in diameter.Furthermore assume that the light-emitting diode be turned on for onemsec and the medium 5 move at a speed of 0.5 d/msec. Then, the pictureelement is formed on the medium 5 as an ellipse whose major axis is 1.5d and whose minor axis is d. With such elliptical picture elements, asshown in FIG. 13A, there remains no blank between the lines and thesteps of an inclined line are unnaturally exaggerated. Thus, high imagequality cannot be attained.

However, according to the fourth embodiment of the present invention,such drawbacks can be substantially eliminated. For instance, assumethat the ratio between the major and minor axes of the elliptical lightemitting aperture 37 of the light-emitting diode be 0.6 d:0.3 d and theratio in diameter between the light emitting aperture 37 and the spot oflight be 0.6:1. Then the spot of light has the major vs. minor axisratio of d:0.5 d. However, since the photosensitive medium 5 moves overa distance of 0.5 d during the time interval 1 msec when thelight-emitting diode is turned on, an almost circular picture element ofa diameter of d is formed on the medium 5.

In summary, according to the fourth embodiment of the present invention,each of the light emitting diodes used has an elliptical light emittingaperture whose major axis is extended perpendicular to the direction ofthe movement of the photosensitive medium, so that an electrostaticlatent image consisting of almost circular picture elements can beformed. As a result, blanks or margins remain between the lines and a"stairstep" imperfection in an inclined line can be minimized. Thus, theimage quality can be remarkably improved.

In the prior art electrophotographic printing devices of the typecomprising a combination of a light-emitting diode array and an opticalfiber array, both arrays are fixedly mounted so that a predetermineddistance may be maintained between a light source and a photosensitivemedium. Meanwhile the optical fibers only serve to transmit the lightrays. As a result, the diameter of a spot of light focused on thephotosensitive medium is almost entirely dependent upon the diameter ofthe optical fiber, so that the diameters of picture elements of acharacter image are always equal. Therefore, as shown at (A) in FIG. 14,the size of a printed character can be varied by changing a dot matrix,but the width of the lines always remain same. That is, it has beenimpossible so far to change the width of lines. In order to increase thewidth of a line segment, spots of light or picture elements are partlyoverlapped in both the vertical and horizontal lines as shown at (B) inFIG. 14. However, to this end, the light-emitting diodes must be turnedon and off many times, so that the power consumption increases and theservice life of light-emitting diodes becomes shorter. In addition, itis apparent that the width of a line segment cannot be reduced less thanthe diameter of the picture element. However, according to the presentinvention, this problem can be overcome as will be described in detailbelow.

In FIGS. 15A and B is shown a fifth embodiment of the present inventioncomprising a combination of a light-emitting diode array 40 and alight-beam transmission-and-convergence means array 39. Thelight-emitting diode array 40 or the light-beamtransmission-and-convergence means array 39 is displaced very finelyrelative to each other so that the diameter of the picture elements canbe varied arbitrarily as shown at (C) in FIG. 14 and subsequently thewidth of a line segment can be arbitrarily varied.

The array 39 comprises a plurality of light-beamtransmission-and-convergence means 2 of the type described previouslywhich are disposed in an array in parallel with the axis of thephotosensitive medium 5 (as best shown in FIG. 15A) in such a way thatthe axes of the light-beam transmission-and-convergence means 2 areperpendicular to the cylindrical surface of the medium 5 (as best shownin FIG. 15B). The array 40 comprises a plurality of light-emittingdiodes 1 disposed in an array in parallel with and in opposedrelationship with the array 39. The light-emitting diode array 40 ismounted on or suspended from a support 41a through piezoelectricelements 42 while the array 39 is mounted on a support 41b.

As described previously, each light-beam transmission-and-convergencemeans 2 not only transmits the light beam emitted from the correspondinglight-emitting diode 1 but also converges or focuses the beam as anextremely spot on the medium 5 as a converging lens and the diameter ofthe spot of light thus focused is variable by changing the distancebetween the light-emitting diode 1 and its corresponding light-beamtransmission-and-convergence means 2. Therefore, according to the fifthembodiment of the present invention, the piezoelectric elements orcrystals 42 interposed between the light-emitting diode array 40 and itssupport 41a are excited or energized so that the distance from thelight-emitting diode 1 to its corresponding light-beamtransmission-and-convergence means 2 is extremely finely adjusted. Ingeneral, the piezoelectric element 42 can produce a maximum strain of 30μm, so that it is best adapted to cause an extremely small displacement.

Thus, according to the fifth embodiment of the present invention, thevoltage impressed across the piezoelectric elements 42 is varied so thatthe diameter of the picture elements can be arbitrarily varied andconsequently a character having broad line segments as shown at (C) inFIG. 14 can be printed. Another advantage of the fifth embodimentresides in the fact that when the diameter of the picture elements isincreased, the picture elements are more overlapped, so thatsufficiently clear printed images can be obtained even when theresolution (dot/mm) is lowered.

So far the light-emitting diode array 40 has been described as beingmoved toward or away from the light-beam transmission-and-convergencemeans array 39, but it is understood that the same effect can beattained by moving the array 39 toward or away from the array 40 as willbe described in detail below.

In FIG. 16 is shown a modification of the fifth embodiment. Thelight-emitting diode array 40 is fixedly secured to the support 41a, butthe light-beam transmission-and-convergence means array 39 is suspendedfrom the support 41a through the piezoelectric elements 42.

In FIG. 17 is shown another modification of the fifth embodiment of thepresent invention. The light-emitting diode array 40 is suspended fromthe support 41a through the piezoelectric elements 42 and the light-beamtransmission-and-convergence means array 39 is suspended from the array40 through the piezoelectric elements 42. Therefore, both the arrays 39and 40 can be moved toward or away from each other. It is quite apparentthat both the modifications shown in FIGS. 16 and 17, respectively, canattain the same effects as the fifth embodiment shown in FIGS. 15A andB.

In summary, according to the present invention, the exposure unit can bemade extremely compact in size by combining light-emitting diodes andlight-beam transmission-and-convergence means such as converging lenses.In addition, the distance between the light emerging end of thelight-beam transmission-and-convergence means and the surface of aphotosensitive medium can be increased as compared with the prior art.As a consequence, the prior art supporting mechanisms which are complexin construction and which must be fabricated with a higher degree ofaccuracy can be completely eliminated, so that the fabrication costs canbe considerably reduced. Moreover, the light-emitting diodes and thelight-beam transmission-and-convergence means can be so combined as toobtain a high resolution and consequently high image qualities. So farwith the prior art electrophotographic printing devices, it has beenimpossible to vary the diameter of the picture elements, but accordingto the present invention, the diameter or size of the picture elementscan be arbitrarily varied, so that the width of line segments can befreely selected.

What is claimed is:
 1. In an electrophotographic printing device of thetype in which electrostatic latent images of characters and the like areformed on the surface of a photosensitive medium, an exposure unitcomprising:a first assembly of a plurality of light-beamtransmission-and-convergence means whose one ends are disposed inopposed relationships with said surface of said photosensitive medium;and a second assembly of a plurality of light-emitting diodes whoselight-emitting apertures are disposed in opposed relationships with theother ends of the light-beam transmission-and-convergence means, saidsecond assembly comprising a plurality of light-emitting-diode-arraychips each consisting of a predetermined number of light-emittingdiodes, said light-emitting-diode-array chips being disposed in azig-zag array in the direction perpendicular to the direction ofmovement of said photosensitive medium, the light-emitting diode at oneextreme end of the light-emitting diode array on eachlight-emitting-diode-array chip in one array or row of the zig-zagarrayed chips being adjacent to the light-emitting diode at one extremeend of the light-emitting-diode array on each light-emitting-diode-arraychip in the other array or row of the zig-zag arrayed chips, so thatsaid first-mentioned light-emitting diode is adjacent to saidsecond-mentioned light-emitting diode, and the pitch between saidlight-emitting diode at one extreme end of the array on each said chipin said first array of chips and said light-emitting diode at oneextreme end of the array on each said chip in said other array of chipsis equal to the pitch between the adjacent light-emitting diodes on eachchip.
 2. An exposure unit as set forth in claim 1, whereina spacer isinterposed between said first assembly and said second assembly.
 3. Anexposure unit as set forth in claim 1, whereinsaid first assembly andsaid second assembly are assembled or integrated into a unitaryconstruction.
 4. An exposure unit as set forth in claim 1, whereinsaidplurality of light-beam transmission-and-convergence means of said firstassembly are disposed in a zig-zag array in the direction perpendicularto the direction of movement of said photosensitive medium.
 5. Anexposure unit as set forth in claim 4, whereinone or more light-emittingdiodes are disposed in opposed relationships with only the area in theproximity of the center of the other end of each correspondinglight-beam transmission-and-convergence means.
 6. An exposure unit asset forth in claim 1, whereinthe axis of the light-emitting aperture ofeach light-emitting diode in the direction perpendicular to thedirection of movement of said photosensitive medium is elongated ascompared with the axis thereof in parallel with said direction ofmovement of said photosensitive medium.
 7. In an electrophotographicprinting device of the type in which electrostatic latent images ofcharacters and the like are formed on the surface of a photosensitivemedium, an exposure unit comprising:a first assembly of a plurality oflight-beam transmission-and-convergence means whose one ends aredisposed in opposed relationships with said surface of saidphotosensitive medium; and a second assembly of a correspondingplurality of light-emitting diodes whose light-emitting apertures aredisposed in opposed relationships with the other ends of the light-beamtransmission-and-convergence means, said second assembly comprising agroup of straight arrays each consisting of a predetermined number oflight-emitting diodes inclined at a predetermined angle with respect toa line or the direction perpendicular to the direction of movement ofsaid photosensitive medium, said arrays being disposed in parallel witheach other; a timing circuit for generating timing signals fordetermining the time at which each light-emitting diode is lighted; acorresponding plurality of drive circuits, each drive circuit includinga corresponding plurality of AND gates each of which is connected inseries to each of the light-emitting diodes in each array; and means forapplying a data signal to inputs of the AND gates for eachlight-emitting diode array and for applying said timing signalssequentially to the other inputs of said AND gates for each array, sothat the light-emitting diodes in each array are turned on and offsequentially.
 8. In an electrophotographic printing device of the typein which electrostatic latent images of characters and the like areformed on the surface of a photosensitive medium, an exposure unitcomprising:a first assembly of a plurality of light-beamtransmission-and-convergence means whose one ends are disposed inopposed relationships with said surface of said photosensitive medium; asecond assembly of a corresponding plurality of light-emitting diodeswhose light-emitting apertures are disposed in opposed relationshipswith the other ends of the light-beam transmission-and-convergencemeans; and means for displacing at least one of said first and secondassemblies toward or away from the surface of said photosensitive mediumby an extremely small distance, said displacing means comprising atleast one piezoelectric element interposed between a stationary assemblysupporting means and said second assembly or between said secondassembly and said first assembly.